The cells involved in bone resorption and bone formation, e.g. osteoclasts, osteoblasts and osteocytes, will be studied (1) in bone tissue maintained in organ culture, and (2) in vivo. Their activity will be assessed by quantitative ultrastructural morphology and these data will be correlated with biochemical data on the same tissue. We will test the effects of regulatory hormones as well as other agents influencing bone metabolism. These studies will form the basis for investigation of the pathology of bone cells and the pathogenetic mechanisms in a number of bone disorders.